BACKGROUND: This study examined the 6-month follow-up outcomes of the Transdiagnostic Sleep and Circadian Intervention (TranS-C), compared to Psychoeducation about sleep and health (PE). METHODS: Adolescents (mean [SD] = 14.77 [1.84] years) with eveningness chronotype and at-risk in at least one of five health domains were randomized to receive TranS-C (n = 89) or PE (n = 87) at a university-based clinic. Primary outcomes were average weeknight total sleep time and bedtime calculated from sleep diary, a questionnaire measure of circadian preference, and composite risks in five health domains. Secondary outcomes were selected sleep diary indices, sleepiness, and self- and parent-reported sleep, parent-reported risks in five health domains. RESULTS: Relative to PE, TranS-C showed treatment effects through 6-month follow-up on only one primary outcome; namely eveningness circadian preference. TranS-C also showed treatment effects on two sleep and circadian secondary outcomes, including the Pittsburgh Sleep Quality Index and sleep-diary measured weeknight-weekend discrepancy in wakeup time. TranS-C did not show treatment effects on self-report or parent-report composite risks in five health domains. PE showed benefit, relative to TranS-C, from posttreatment to 6-month follow-up for reducing parent-reported behavioral health risk (secondary outcome). CONCLUSIONS: In at-risk adolescents, the evidence supports the TranS-C treatment effects over six months on improving sleep and circadian functioning on selected outcomes but not on reducing risk in five health domains.